ADEM Publications, Presentations and other output

ADEM--18-106

Title:

Water sensitivity and microporosity in organosilica glasses

Author(s)

Dral, Petra; ten Elshof, J;

Identification

ISBN

978-90-365-4400-9

DOI

10.3990/1.9789036544009

Publication date

2017-10-27

Number of pages

124

Full text

not available

Abstract

In this dissertation the water sensitivity and microporosity of organosilica glasses are studied. The research focuses on fundamental material understanding, but stands in close relation with the industrial application of organically bridged silicas as molecular sieving membranes. Chapter 1 presents a systematic study on the influence of monomer connectivity, network flexibility and hydrophobicity on the hydrothermal dissolution of organosilicas. Bond strain appears to significantly increase the tendency to dissolve under hydrothermal conditions. The stabilizing influences of increased connectivity and hydrophobicity were found to be weak. Chapter 2 zooms in on subtle effects of condensation reactions in ethylene-bridged silica when kept at temperatures up to 300 °C. An explanation is presented for the previously not understood problem of slow flux decline in industrially employed organosilica membranes over periods of months to years. The common assumption that a stabilized structural state is reached after treatment at 250-300 °C for a few hours is shown to be incorrect. Chapter 3 presents a post-treatment to solve the subtle material instability reported in Chapter 2, involving exposure to in-situ synthesized HCl gas alternated with heat treatments at 150-300 °C. Treatment with HCl was found to predominantly catalyze hydrolysis of siloxane bonds, enabling network optimization via iterative bond breakage and reformation. Chapter 4 presents a new method based on vapor thermogravimetry and gas pycnometry for characterization of micropores <1 nm with increased accuracy as compared to conventional adsorption isotherm analysis. Main advantages of the demonstrated method are that diffusion limitations due to cryogenic temperatures are eliminated, adsorption is studied with non-polar gases, micropore cavity sizes are probed separate from micropore entrances and data can be interpreted in a straightforward fashion without requiring theoretical models on molecular behavior. Chapter 5 presents a systematic study on the micropore properties of a series of organosilica materials based on the method reported in Chapter 4. The known classification of 1) short or rigid organic bridges that open up the pore structure, 2) longer and more flexible bridges that cause pore filling and 3) terminal organic groups that reduce pore formation is further specified. The incorporation of any organic group in the silica network increased the dispersity in micropore entrance sizes as compared to inorganic silica in the probed size range. A critical discussion is given of the commonly accepted ‘spacing concept’ of organic bridges.